Diffusion - possible to have too much?

[Josh0311]

A couple points to preface my question:
1. I don't have money right now to hire a consultant.
2. I'm not looking for a permanent fix, just a reasonably effective band-aid.
3. I'm not in a position to entertain the best approach, but a balance between better & inexpensive
4. Given the heinous attributes of our auditorium it would be difficult to down-grade it.

Whew, having said all that...
Is it possible to have too much diffusion? (acoustically, not financially) I'm in the process of researching some intermediate fixes to our auditorium, one of which is diffusion (currently none). At some point I'll need to settle on a design for diffusers, but my immediate concern is whether an error in quantity on the liberal side will create new problems. Given some research elsewhere, I'm looking at using Sono-Tubes (tubular concrete forms) split in half or thirds as the core product. Not sure yet on fill or mounting.
Auditorium is very reverberant, all hard surfaces (including floor), several large parallel walls, seats 500, cathedral ceilings - 25' at peak. Yea, I know - sounds more like a gymnasium.

Thanks in advance for the help...

[cmchamp]

Couple of points to preface my answer:
1: Do you have the money to do it again?
2: If you've got the time to band-aid, do it right the first time.
3: Are you in a position to entertain making the room worse?
4: You'd be surprised what miss-application of small space acoustics can do to a larger space. (even a 500 seat auditorium is considered a small space, but it's much larger than a home theatre or home studio)
5: How do you know you can't afford an acoustic consultant?

That all said, proper acoustical treatments, be they diffusion, absorption or both, requires proper design to be most affective. In addition to the physical dimensions of the space, finish treatments, etc., actual acoustic measurements (sound recording or sound measurement devices) need to be utilized. Simply placing 1/3rd or 1/4 cuts of Sonotubes may not do much, if anything, at the frequencies requiring attention.

So, if you're up for a learning experience for the group, and are willing to be patient, we can work on an acoustical design to take care of both flutter echoes in the room as well as the assumed required LF correction.

I read much of the same home made home studio acoustic threads you do, and unfortunately, there is little evidence of applied acoustics and applied mathematics behind the postings. "It looks cool and the room sounds better (different)" seems to be the main themes sounding on those threads.

Churches, auditoriums and even gymnatoria are different beasts all together. My mentor (I studied acoustics from him for about 8 years, he's had me on my own for the past 2-3) is well known for his large cathedral work. He does the rare small room, which he defines as anything below 1500 seats, but unfortunately, small rooms can cost more per seat to correct.

Let me know if you're interested in the "Acoustics Self-Help" study.

C.

[cmchamp]

By the way, I don't plan on revealing any "Trade Secrets" because there really aren't any. Acoustics is not an "exact" science, nor is it rocket science or geophysical astronomy. Once the understanding of the mathematics and physics is in place, which I hope to possibly accomplish through this, then a greater understanding of the nuances of acoustics can be attained.
We are not to 'promote' ourselves, however, I am an acoustical consultant, but it is not my primary source of income and support for my family.
I do 2-3 gratis projects a year, and this can be one that an entire community can learn from, as well as myself.
C.

[Josh0311]

Well Cory, it sounds like you are significantly further down the road of knowledge than I...so I will defer to your judgment. Your prefacing questions are legitimate, and I acknowledge there may be solutions I'm not considering or aware of.
Beside the tangible need of a corrected room, I'm personally very interested in the topic and would certainly appreciate the chance to learn more. Count me in on the study. Luckily, I don't have anyone breathing down my neck on this, so speed isn't critical. Let me know what you need from me. Thanks for your time, I very much appreciate it.

[Brad Weber]

I agree with Cory, it is the proper application of solutions to the specific situation and not just what types of solutions are used. I also wholeheartedly agree with Cory's comments regarding the application of small room acoustics (modal controlled) to large spaces (reverberant controlled) and vice versa, I've even been involved in projects that had multiple acoustical consultants who each specialized in different areas.

I will also add that just like many other aspects of any space, proper acoustics is a function of the desired result and environment. Rooms do not have inherently bad or good acoustics, the acoustics should be in keeping with their initial intended use and the desired environment, which will vary from one application to another.

[tedanderson]

Here is some food for thought-

Ever been in a carpet store or clothing store and noticed how unusually "quiet" it is in there?

You could probably duplicate the same results if you used old donated clothes and scraps from carpet installers. Then if you cut up everything into washcloth sized pieces and packed it loosely in burlap sacks, that would start to help with the acoustical treatement by absorbing the sound on some of those reflective surfaces.

When it comes to diffusion, I am not an expert on the best "cheap" materials to do that but imagine that if you fill those same burlap sacks with disposable plastic cups (loosely scattered inside as to not have them stacked), that would change your flat surface to something different. I am not sure that the results will be desireable but your sound system would no longer be reflecting off of a flat hard reflective surface.

[cmchamp]

Ted
Carpet and clothing stores seem so quiet because all of that soft stuff absorbs the frequencies we are most sensitive to.
I don't know if you intended your subsequent comments in jest, but that's what we call in another group "Throwing dead cats on the wall."
Sure, soft stuff is going to absorb the sound, but what frequencies and why is what needs to be addressed.

So, Josh, let's start with the gross dimensions of the room.
Length (x)
Width (y)
Wall Height (z)
Ceiling Height. (z)

X/Y Location of the pulpit or speaking location and locations of mains.

Then, I need to know if you'll be able to send me a CD recording of some balloon pops from a pair of omni-directional microphones.

I will fully disclose every calculation, and might even provide some of the spreadsheets I use. Don't anyone go all crazy now and think that they're all of a sudden going to be an acoustical consultant from this exercise, because that's not going to happen. We all will learn something from this, including myself, I'm sure, as I try to explain the where's and why's of my methods and processes.

After I have your gross dimensions, I will be able to provide estimates on a couple of things such as room modes at the listening positions standing and sitting as well as a sample recording estimating the acoustics of the room. I don't do the recording estimate as a snake oil trick, just so that I can attempt to estimate what your room sounds like and so that we can listen as that sound changes depending on the type of acoustical treatments utilized and where they are placed.

I look forward to your information, and will get at the preliminaries as soon as I have 2 grad school papers finished.

C.

[Brad Weber]

Just to clarify, what Cory is suggesting addresses the room acoustics but not ambient noise levels or sound isolation, those get into absolute rather than relative levels and thus require accurate measurement of absolute levels. Also, a balloon burst can give you data on the room itself but will not include a sound system's interaction with the room and will not allow measurement of related criteria such as %ALCONS or STI since those are both room and system dependent. This is not at all being critical of Cory's suggestion, it is simply indicating how complex the issue can be.

One common problem with many room acoustics projects, especially those with very bad conditions, is what I call the onion effect, as in there can be multiple layers. One problem may be effectively masked by another possibly more apparent problem. But address the obvious problem and suddenly the previously 'hidden' problem can appear. One very common such occurrence is having a very reverberant or live room masking specific echoes or reflections, but once the overall reverberation of the space is reduced these specific problematic reflections can become more noticeable. This is they type of thing I think Cory was referencing when he was addressing developing more effective treatments rather than simply general treatments.

[tedanderson]

I don't know if you intended your subsequent comments in jest, but that's what we call in another group "Throwing dead cats on the wall."

Yeah, I figured there was terminology for that. When I was in high school our crazy band director did something similar and we (the students) noticed a big improvement. It wasn't very scientific but as broke as our school system was, anything was better than nothing. And the original poster's situation brought me back to that.

[Josh0311]

I'll get those dimensions once I get into the office today...until then. Here's a few more facts for those that may be interested:
1. Acoustic drums, 5' plexi shield, rear absorptive surround
2. Guitar amp (1), plexi shield
3. Bass on DI (Sansamp), no stage amp
4. No wedges, all IEM
5. FOH runs in the low 90's (db weighted C)

Anecdote - part of our stage was made "portable" before I got here (very little bracing or mass). I had the subs placed next to the stage, but the LF caused the stage to resonate which was transmitted up one of the mic stands and into an open mic - nasty feedback.

[cmchamp]

So far, so good.
Depending on the construction of the 'portable' portion of the stage, it can be bulked up and made more rigid with proper cross bracing.
I'll await your further comments and postings.
C.

[[Director] drew ryan]

I just witnessed the strangest thing I have seen in construction in a while. the entire balcony seating is make out of Styrofoam. they had large blocks stacked on each other and framed in the fronts and poured concrete. what a trip. I am sure that there will be little if any audio get through the balcony into the atrium. I wonder if that same thing can be applied to the "portable" part of the stage???